This invention relates to electrical detective circuits for railway points and crossings.
It has been proposed to provide an ac generator to supply energy across a points or crossings junction. A capacitor is connected across the junction to form a tuned circuit, at the generator frequency, with the inductance of the lengths of track between the junction and the intersection of the tracks.
The resultant voltage arising in these lengths of track is used to supply second circuits located in the converging entries of the points or crossing. Each second circuit comprises a short circuit across the tracks and a second capacitor connected across the tracks at a position between the short circuit and the points or crossing. The second capacitor forms a tuned circuit at the generator frequency with the inductance of the short circuit and of the lengths of track between the short circuit and the second capacitor.
When a train axle passes the short circuit the axle shunts the second circuit and its tuning is disturbed. This provides a means of detecting the presence of the axle.
The circuit described above suffers from the disadvantage that it cannot be overlapped with a jointless track circuit such as the "Aster" track circuit. The reason for this is that the short circuit described above will impose a non-detective zone between the points or crossing circuit and the jointless track circuit.
To overcome the above disadvantage, it has been proposed to replace the short circuit and capacitor with a frequency selective low impedance link across the tracks. The link comprises a capacitor forming a tuned circuit with the inductance of an inductor connected in series therewith. The disadvantage of this arrangement is that an axle passing through the entry only gradually shunts the tuned link circuit with the result that an axle, particularly if it only provides a high resistance shunt, can be some distance from the link before it is detected.